A multiconfigurational self-consistent field study of the thermal decomposition of methyl azide

摘要

Thermal decomposition of methyl azide has been studied computationally by using the complete active space self-consistent field (CASSCF) method and Moller-Plesset theory using the CASSCF wave function as the zeroth-order wave function (CAS/MP2). The calculations have been performed in conjunction with the 6-31G~* basis set. The reaction is predicted to occur in two steps via nitrene intermediate: (1) CH_3N_3 -> CH_3N + N_2; (2a) CH_3N -> H_2 + HCN, (2b) CH_3N -> H_2CNH. The rate-limiting step is the N_2 extrusion (1), being a competitive mechanism between a spin-forbidden path and a spin-allowed one. The calculated energy barrier height for both processes is found to be isoenergetic, #DELTA#E = 41 kcal/mol, where #DELTA#E represents the difference between the energy at the minimum on the singlet state surface of methyl azide and the energy at the minimum energy crossing structure (ISCl) or the singlet transition state (TSl) for the spin-forbidden path and the spin-allowed one, respectively. The nitrene intermediate formed in step (1) can undergo two parallel reactions: (2a) decomposition in H_2 and HCN, and (2b) isomerization to methyleneimine. H_2 extrusion from the imine generated in step (2b) has been studied as well. Two high energy transition state have been found for 1,1-H_2 and 1,2-H_2 elimination from methyleneimine, respectively.